This invention generally relates to vent dampers and more specifically is directed to a vent damper having a temperature responsive element for reducing vent heat loss.
Conventional domestic heating systems include a flue, or exhaust stack, which may incorporate a damper for the reduction of heat loss. These dampers may be manually or automatically operated. During periods in which the furnace is off, the damper closes the flue to prevent heat loss up the flue through convection. In addition, the damper is intended to trap the heat of the furnace after furnace shut-off so that this residual heat is available for delivery to the space being heated rather than lost up the flue. However, damper operation must be delayed until the noxious combustion products are purged from the fire box. If this delay is electrically or mechanically actuated, the damper closing may vary with ambient and/or operating conditions. If the closing is temperature-responsive and does not occur until the flue temperature falls to room ambient temperature, too much heat is sacrificed because the combustion products will long since have been purged.
According to a recent government study, damper flue closure when the furnace is off accounts for approximately 20% of energy saved by the damper. A more substantial contribution to heat loss reduction by the damper, the remaining 80%, occurs immediately after furnace turn-off due to damper closure. Thus, substantially more heat is lost over a relatively short period of time following furnace turn-off than over the longer periods between furnace operation because of the high furnace operating temperatures.
Conventional dampers are generally either motor driven or temperature responsive. The former damper employs an electric motor which is controlled by a temperature sensor for mechanically closing the flue following furnace shut-off. This damper approach is expensive in the original cost of components, cost of installation and cost of repair and maintenance.
While the temperature responsive element type of damper generally offers the advantages of lower cost and reduced complexity, this approach too suffers from performance limitations. The typical vent damper employing a bimetallic actuator as the temperature responsive element acts to close the flue only when the temperature of the actuator reaches ambient. This permits the escape of substantial amounts of residual heat between furnace operating cycles, considering a system operating at a normal six cycles per hour. This loss is due to the inability of the bimetallic actuator to completely close even though the firebox is purged of combustion products because the presence of residual heat between cycles maintains the bimetallic actuator open just enough to permit the loss of residual heat.
U.S. Pat. No. 3,228,605 to Diermayer et al. discloses an automatic flue damper having a temperature responsive element consisting of a slotted bimetallic damper plate normally extending across the flue duct. The plate is divided by the slots into longitudinal strips with one end of each strip fixedly attached to the duct with the strips located in a common plane at normal temperature. With an increase in temperature, the free end of each strip moves arcuately away from a fixed duct element and gradually opens a passage for the hot flue gases. In this configuration, the bimetallic strips will not assume a completely closed position across the duct until ambient temperature is reached following burner shut-off resulting in the loss of residual heat via the flue.
U.S. Pat. No. 3,510,059 to Diermayer et al. discloses a flue damper arrangement intended primarily to provide for rapid vent opening following burner ignition. In this approach, the circular flue conduit is divided into four 90.degree. sectors formed by partitions extending across the conduit perpendicular to the direction of gas flow. To each partition is connected a damper section in the form of a one-quarter circle which includes elongated, transversely juxtaposed strips of laminated, bimetallic material extending substantially in a common plane transverse to the conduit axis when cold and defining longitudinal slots between each such quarter circle sector. One of the narrow edges of each strip is fixedly fastened to one wall of the corresponding passage section and the other narrow edge is free to move about the fixed portion in response to temperature change. When the damper section is deflected by rising temperature, a flow passage opens between the aforementioned longitudinal edge and the closely juxtaposed partition at a rate approximately proportional to the increase in the angle of deflection. This configuration allegedly allows for the rapid opening of the flue passage in response to increasing exhaust gas temperatures, but fails to address the problem of reducing heat loss following burner shut down.